Dichlorophosphazene trimer is ordinarily purified by separating the cyclic trimer from linear oligomer. However, the "purified" trimer contains trace amounts of catalytic impurities, sometimes called protic impurities (i.e. materials capable of acting as proton doners). These catalytic or protic impurities include, for example, water, methanol, ethanol, isopropanol, alkyl amines and other basic materials, hydrochloric acid, and hydrolyzed cyclic and linear trimer including phosphoramidate. The term "trace amounts" refers to amounts of impurities on the order of parts per million (hereinafter PPM), e.g. 10 to 1,000 PPM.
These trace amounts of catalytic impurities have a catalytic effect during polymerization. Since the amount of these in each batch of trimer differs, the product of a polymerization run is ordinarily not reproducible.
Some of the catalytic impurities are readily removed. For example, water is readily removed by drying. Others of the catalytic impurities, e.g. hydrolyzed trimer, are very difficult to remove under normal conditions.
Because of the desirability of removing catalytic impurities and because of the difficulty involved as described above, work has been carried out directed to removing all such impurities, i.e. to purification to the extent of producing non-self-polymerizing cyclic trimer, i.e. cyclic trimer which does not polymerize in the absence of added catalyst even at temperatures as high as 300.degree. C.
Non-self-polymerizing trimer has been described in Colclough and Gee, Journal of Polymer Science: Part C, No. 16, pp: 3639-3642 (1968). Colclough et al. produce this by a sequence of operations involving recrystallization, refluxing at 130.degree. C. over BaO in an atmosphere of dry nitrogen, passage of the trimer vapor at 130.degree. C. over phosphorus pentoxide, followed by repeated sublimations. This is a very complicated many step procedure.
Non-self-polymerizing trimer can also be prepared by the method described in Fieldhouse et al. U.S. Pat. No. 4,327,063. In the process of this patent, trace amounts of protic impurities are removed by treating cyclic dichlorophosphazene trimer with boron trihalide under specified temperature and pressure conditions.
The invention herein offers an alternative to the process of U.S. Pat. No. 4,327,063. While relying on phosphorus pentoxide (P.sub.2 O.sub.5) like Colclough et al., the invention herein is directed to a process which is much less complicated and requires less equipment and time than that of Colclough et al.